Fractional flow reserve (FFR) is a metric that has been used to indicate a functional significance of a coronary artery stenosis. It has been used with interventional cardiology procedures to measure pressure differences across the coronary artery stenosis to determine whether a stenosis impedes blood flow and hence oxygen delivery to the heart muscle, which may result in myocardial ischemia. FFR is a ratio of a pressure Pd behind a stenosis relative to a pressure Pin before the stenosis, or Pd/Pin, which is a ratio of maximum blood flow distal to a stenotic lesion to normal maximum flow in the same vessel. FFR is performed during coronary catheterization. For this, a catheter is inserted into the femoral or radial arteries using a sheath and guidewire. FFR uses a small sensor on the tip of the wire to measure pressure, temperature and flow to determine the exact severity of the lesion. This is done during maximal blood flow (hyperemia). A pullback of the pressure wire is performed, and pressures are recorded across the vessel.
Instantaneous wave-free ratio or instant flow reserve (iFR) is another measurement that can be used to indicate a functional significance of a coronary artery stenosis. iFR is defined as the ratio of Pd to Pin over a specific period in diastole referred to as “the wave-free period.” During this wave-free period, the competing forces (waves) that affect coronary flow are quiescent, meaning pressure and flow change in a similar way, which makes their ratio (i.e. resistance) almost constant, compared to the rest of the cardiac cycle. iFR is also performed during cardiac catheterization using invasive coronary pressure wires which are placed in the coronary arteries that are to be assessed. Both FFR and iFR are values in a range of zero (0) to one (1), where a higher value indicates a non or less-significant stenosis and a lower value indicates a more significant stenosis. Unfortunately, both FFR and iFR are invasive procedures and thus are susceptible to complications ranging from infection to death.
FFR-CT is a non-invasive procedure used to provide an estimation of FFR using a computational fluid dynamic (CFD) simulation on coronary geometry that is extracted from a non-invasively cardiac CT study. Unfortunately, CFD based FFR estimation requires a complicated dynamic simulation with many transitional effects occurred during cardiac contraction and relaxation and a complicated model with many uncertainties which, may lead to high estimation error.